Strike Of Strata Research Articles

Coupled effects of impact and orogeny: Is the marine Lockne crater, Sweden, pristine?

Abstract— Our current understanding of marine‐impact cratering processes is partly inferred from the geological structure of the Lockne crater. We present results of a mapping campaign and structural data indicating that this crater is not pristine. In the western part of the crater, pre‐impact, impact, and post‐impact rocks are incorporated in Caledonian thrust slices and are subjected to folding and faulting. A nappe outlier in the central crater depression is a relic of the Caledonian nappe cover that reached a thickness of more than 5 km. The overthrusted crater is gently deformed. Strike of strata and trend of fold axes deviate from standard Caledonian directions (northeast‐southwest). Radially oriented crater depressions, which were previously regarded as marine resurge gullies formed when resurging seawater erosively cut through the crater brim, are interpreted to be open synclines in which resurge deposits were better preserved.The presence of the impact structure influenced orogenesis due to morphological and lithological anomalies of the crater: i) a raised crater brim zone acted as an obstacle during nappe propagation, (ii) the occurrence of a central crater depression caused downward sagging of nappes, and (iii) the lack of an appropriate detachment horizon (alum shale) within the crater led to an enhanced mechanical coupling and internal deformation of the nappe and the overthrusted foreland. Preliminary results of 3‐D‐analogue experiments suggest that a circular high‐friction zone representing the crater locally hinders nappe propagation and initiates a circumferentially striking ramp fault that delineates the crater. Crustal shortening is also partitioned into the crater basement and decreases laterally outward. Deformation of the foreland affected the geometry of the detachment and could be associated with the activation of a deeper detachment horizon beneath the crater. Strain gradients both vertically and horizontally result in non‐plane strain deformation in the vicinity of the crater. The strain tensors in the hanging and foot walls may deviate up to 90° from each other and rotated by up to 45° with respect to the standard regional orientation. The observed deflection of strata and fold axes within the Lockne crater area as revealed by field mapping is in agreement with the pattern of strain partitioning shown in the analogue models.

Clay Mobility in Ridge Route Landslides, Castaic, California

Substantial earth movements along clay-bearing bedding planes have produced landslides in road cuts along the Los Angeles-San Francisco superhighway, Interstate 5, where it crosses the western extension of the San Gabriel Mountains in southern California between Castaic and Gorman. Such slides occur where the strike of the clay-bearing strata in road cuts is parallel with the highway, and the dip is toward the roadway at an angle less than the slope of the cut. Slides also occur on dip slopes. Thick sandstone beds have moved on wet clay-bearing members that range from a few inches to as much as 20 ft thick. Laboratory studies show that slide-prone montmorillonite and illite are abundant minerals in the clay-bearing strata. Further, when the water content of the clay that contains these minerals exceeds 50 percent, the cone shear resistance drops to 0.001 to 0.1 tsf, whereas at 20-35 percent water content, the shear remains in excess of 1.0 tsf. Where the strike of strata is normal to the direction of the highway, landslides either have not been observed, or are minor features. In places where the dip is away from the roadbed, although the strike is parallel with the highway, slides ordinarily are not observed. A monumental highway fill across West Liebre Gulch merits special attention. South of the gulch, strata strike approximately normal to the line of the freeway and dip north toward the bottom of the gulch. Clay-bearing zones lie just beneath the surface. Apparently these caused an old landslide, formed long before construction was contemplated, which extended across the line chosen for the freeway. Fill, emplaced when the freeway was constructed, suffered extensive slide action when the old slide, partly buried beneath the fill, was reactivated following heavy rains both in 1967 and in 1969.

A Floating Compass to Measure Dip and Strike of Strata

A Floating Compass to Measure Dip and Strike of Strata